When an HID reaches stable arc discharge, a discharge positive column of a coplanar discharge characteristic area is a section of typical high-pressure gas isothermal plasma. In a discharge lamp tube, the concentration of gas atoms is high, there are few free paths of electrons, the electrons highly frequently form elastic collisions with the gas atoms, and the energy of a modulated electric field is easily transmitted to the gas atoms through the electrons, thereby causing gas pressure waves under a frequency the same as that of the external modulated electric field. The pressure waves of discharge gas in the discharge lamp tube may form standing waves, that is, acoustic resonance is produced. A frequency range is very wide, in which a working frequency common for an electronic ballast is included.
When the HID lamp works under a modulated frequency, an acoustic resonance phenomenon arises, which causes influence on the application of a frequency-modulated electronic ballast. The so-called acoustic resonance phenomenon refers to discharge arc instability presented when the HID lamp is driven by a frequency-modulated current. When the HID lamp works within a frequency range of 5-700 KHz, obvious light output fluctuation accompanying with current and voltage fluctuation and arc cambering and shaking will occur in multiple frequency bands. When the frequency is reduced to the lowest instable frequency, an arc may be extinguished, and even an arc tube is exploded.
The acoustic resonance phenomenon manifests as discharge arc instability, cambering and shaking, arc breakage under a serious condition and even arc tube explosion, there are many factors for the occurrence of resonance, such as a shape and size of the discharge tube, the pressure and temperature of the gas in the tube and the service life of the lamp, different batches of lamps produced by different manufacturers have different acoustic resonance frequency ranges, and more importantly, along with the prolonging of service life of the lamps, acoustic resonance points will be changed to a certain extent. Therefore, it is very difficult to solve the problem of acoustic resonance.
A low-power discharge lamp is small in size and high in resonance frequency, a low-frequency square wave driving mode can be used for effectively solving the problem of acoustic resonance. In the ultraviolet curing industry, due to a large curing area and a high energy density requirement, the power of the lamp tube is ranged from several kilowatts to tens of kilowatts. The size of the lamp tube is undoubtedly increased, and the resonance frequency is reduced accordingly. Therefore, the acoustic resonance phenomenon still arises in an actually used working frequency band of the electronic ballast, which causes the problem of reduction in radiation output by ultraviolet rays, and meanwhile, the ballast is damaged by an instable acoustic resonance current.
At present, there are many acoustic resonance detection methods, one of which is to detect a current during the occurrence of acoustic resonance, and a theoretical foundation for such an acoustic resonance detection method is that the current of the lamp changes during the occurrence of acoustic resonance. An experiment shows that, when acoustic resonance occurs during the work of the HID lamp under the modulated frequency, the current of the lamp mainly includes three frequency components: the first is a frequency-modulated current frequency provided by the ballast; the second is a ripple frequency; and the third is a current generated by acoustic resonance. A detection circuit filters the former two currents with the acoustic resonance current left, and amplifies the acoustic resonance current to judge whether the HID lamp is in an acoustic resonance state or not. Like the other detection methods, the method is very complex and low in practicability.